The present invention relates to apparatus and methods for generating traps in digital document preparation or prepress operations.
A page in an electronic document may include various types of objects, including text, line art, and images. Electronic documents are generally created by computer programs (also called application programs or simply applications) that may be executed by a user on a computer to create and edit electronic documents and to produce (directly or indirectly) printed output defined by the documents. Such programs include the Adobe Illustrator.RTM. and Photoshop.RTM. products, both available from Adobe Systems Incorporated of San Jose, Calif. Objects in electronic documents may be represented in vector form, raster form, or in hybrid forms.
A color page in an electronic document includes information about the colors used in the page. Colors are defined in accordance with a "color space", which provides a data representation of a range of colors in terms of basic color components. The specific color components will vary depending on the color system used. For example, in the CMYK color system, colors are represented as combinations of cyan (C), magenta (M), yellow (Y), and black (or "key") (K).
To create a physical page printed with inks, data representations of colors are used to create color separations. This is general done by computer programs running on general or special purpose systems. Generally, each color separation used by an output device will correspond to a color component of the color system used by the device. For example, data representations of colors in output generated for an imagesetter using a CMYK color system will be used to create color separations for cyan, magenta, yellow, and black, with each separation indicating regions where the corresponding ink should be applied, as well as the amount of ink that should be applied.
Misregistration or inaccuracies in the physical placement of two or more colors with respect to each other on a printed sheet in printing can cause unintentional gaps or overlaps at edges of color regions on an output page. Misregistration may occur for a variety of reasons relating to the performance of people, machines, and materials.
To minimize the effects of misregistration, a technique known as trapping adjusts the shapes of color regions by spreading (expanding) some color regions to prevent gaps, and choking (contracting) other color regions to prevent overlaps. The adjusted areas into which inks will be spread or from which inks will be contracted are referred to as "trap regions", and trapping also entails determination of the amount of ink to be applied to the trap regions for each affected separation.
In the case of a spread, the amount of spread desired for a particular printing job may generally be set to a value--the trap width--specific for the job in view of the specific paper, inks, press, and required quality of the job. Whether any spreading occurs at all at a particular location for one ink with respect to another ink is determined separately, and depends, among other things, on whether the other ink is present at the location. When it has been determined that spreading is to occur, there remains the question of how to allocate the trap width is between the two colors.